Chronic pain represents one of the most significant societal burdens in terms of the number of Americans affected, its impact on the health care system and lost productivity. Classical opiates, such as morphine, remain the gold standard of care for the management of moderate to severe post-operative and cancer pain as well as for the treatment of chronic non-malignant and inflammatory pain. However, the long-term use of mu opioid receptor (MOP) agonists such as morphine, in the setting of chronic pain, is limited by the development of tolerance and physical dependence. Opiate tolerance is the gradual loss of drug potency or efficacy, and reduced duration of action. Tolerance is frequently accompanied by physical dependence and in some cases by addiction. The opioid receptor subfamilies include mu, delta, and kappa opioid receptors (MOP, DOP, and KOP). While it is clear that morphine-induced analgesia is mediated by MOP activation, the role of DOP in analgesia remains unclear. It has been reported that morphine-induced analgesic tolerance in acute pain is reduced upon administration of DOP antagonists and in mice lacking functional DOP. Surprisingly, there are no studies regarding the role of DOP in the development of morphine-induced analgesic tolerance in chronic pain. We propose that targeting both the MOP and DOP will reduce morphine-induced analgesic tolerance in chronic pain. The concept that functional and physical interactions between MOP and DOP play a key role in the development of morphine-induced analgesic tolerance during chronic pain provides a novel target through which modulation of MOP-DOP interactions may improve the side-effect profile of morphine and other MOP ligands. The proposed experiments will test the hypothesis that pretreatment with DOP antagonists or disruption of the MOP-DOP heteromer will result in an attenuation of the analgesic tolerance that develops after repeated morphine injections during chronic pain. We also propose that morphine-induced analgesic tolerance is mediated by increased DOP expression and function as well as by increased MOP-DOP heteromer abundance at the primary afferent, spinal cord and/or at brain areas implicated in opioid control of nociception such as the midbrain periaqueductal gray (PAG). In Specific Aim 1 we will conduct biochemical and behavioral analyses to investigate the role of MOP-DOP interactions in the attenuation of morphine- induced analgesic tolerance by DOP antagonists in a chronic inflammatory pain model. In Specific Aim 2 using in vitro recordings, we will characterize the role of DOP and MOP in the attenuation of morphine-induced analgesic tolerance in a chronic inflammatory pain model. The outcomes of the present studies will have a sustained, powerful impact on the fields of the biology and pharmacology of opioid receptors with the prospects of novel, safer and more effective pharmacotherapeutic strategies for the treatment of chronic pain. In fact, since MOP agonists are already widely used in the clinic, ameliorating their negative side-effects and potentiating their analgesic effects have the potential to rapidly benefit chronic pain patients,